For example, health insurers have been found to have done such things as kick cancer patients off insurance as a pre-existing condition, based on previous general practitioner visits. "Oh, we, and you, didn't know it was cancer at the time, you were just sick. But since you were sick shortly after getting insurance, you most likely already had cancer. Therefore it was a pre-existing condition, you didn't disclose it, whether or not you knew about it, and we are terminating your coverage back to the first physician visit about your illness".
Or "We are refusing to cover the costs of the ambulance/air ambulance ride that ferried you to the trauma center after your MVA, because that ride wasn't pre-approved by a call to the insurer".
Both of the above examples happened. I'm not sure what should have happened in either case, but I'm pretty sure in the latter it wasn't, "Hello, Blue Cross? This is John, I'm a paramedic working on one of your patients who was hit by a truck. We would like to fly him to the hospital due to his extensive multisystem trauma but we need your approval. His name? Hang on, let me find his wallet. No, that's Smythe, S-M-Y-T-H-E, sorry, it's a bit loud with the jaws of life in the background... Yes, I can hold for a nurse consult..."
Health insurance in the US is amortization of costs, and has little to do with risk pooling and actuarials.
We've had great mechanism to address this, namely insurance and savings. The underlying principle here is planning ahead, and sacrificing some of today's sweet instant gratification in exchange for protection tomorrow should you need it. This is a concept that people have seemed to lose sight of, so we're now seeing people want protection today even though they made no sacrifice yesterday. Those people with zero foresight have now ruined it for the people that did plan ahead.